Abstract : Ultrastructural and biochemical changes in lysosomes of rat liver following exposure to ionizing radiation were studied. A marker for acid phosphatase was used to visually correlate the progressive changes in lysosomes with the cellular necrobiotic process postirradiation. The earliest observable change in the lysosomes and/or lysosomal enzymes corresponding with the sequence of fine structural alterations following irradiation suggests that radiation labilizes the lysosomal membrane resulting in a release of enzymes responsible for cell damage. The concentration of the lead phosphate reaction product indicated that the initial action on the lysosome is evidently a 'build-up' of hydrolytic enzymes within 2 hours after irradiation followed by a gradual release of the marked enzyme 2-24 hours postirradiation as noted by decreased enzyme concentration within the lysosomes. The release of the enzyme appeared to be directly related to an increasing cellular necrobiosis following irradiation. (Author)